In recent years, a device for sea resources exploration and an ocean-bottom earthquake sensor network for early detection of earthquake are widely used and the needs for power supply means for supplying an electric power to these devices increase. These devises operate in seawater. Therefore, if an electric power can be transmitted wirelessly, it is desirable to use the power supplying means using a wireless electric power transmission technology. That is because when a metal plug for electric power supply is used in seawater having an electrical conductivity of approx. 4 S/m, there is a possibility that a short circuit may occur in the metal plug for electric power supply. When the electric power is supplied to the device for sea resources exploration or the ocean-bottom earthquake sensor network mentioned above, at least one of a transmission-side electric power transceiver and a reception-side electric power transceiver is not fixed and floats in seawater or on seawater. Therefore, a distance between the power transceivers has to be approx. 10 cm or more to avoid collision or contact even when accuracy of attitude control of the transmission side device or the reception side device is taken into account. Therefore, it is necessary to keep a distance of approx. 10 cm or more between the transmission side device and the reception side device.
With respect to a technology for wirelessly transmitting and receiving electric power or a system for wirelessly transmitting and receiving electric power, in patent literature 1, a system using an electromagnetic induction phenomenon is described and in patent literature 2, a device using a microwave is disclosed.
However, in recent years, a wireless electric power transmission system using a resonance phenomenon (magnetic field resonance) of a magnetic field energy described in patent literature 3 is proposed. This magnetic field resonance method is a technology in which a coil with high Q value is used, a mutual inductance between a power transmitting unit and a power receiving unit is improved by resonating the coils at the same frequency (resonance frequency), and a transmission distance can be made long. Therefore, the magnetic field resonance method has a feature in which both a long electric power transmission distance and a high electric power transmission efficiency can be realized simultaneously. Further, when a method using a microwave is used, the electric power transmission distance can be made long but the electric power transmission efficiency is remarkably low compared with the resonance method. By an electromagnetic induction method, the electric power transmission efficiency that is approximately equal to that of the resonance method can be realized. However, the electric power transmission distance is remarkable small compared with the resonance method. Therefore, it is expected that the magnetic field resonance method is applied to a mobile object such as an electric vehicle or the like requiring a large electric power or a mobile device as a new charge technology.